MRI is a non-invasive diagnostic technique which produces well resolved cross-sectional images of soft tissue within an animal body, preferably a human body. This technique is based upon the property of certain atomic nuclei (e.g. water protons) which possess a magnetic moment [as defined by mathematical equations: see G. M. Barrow, Physical Chemistry, 3rd Ed., McGraw-Hill, N.Y. (1973)] to align in an applied magnetic field. Once aligned, this equilibrium state can be perturbed by applying an external radio frequency (RF) pulse which causes the protons to be tilted out of alignment with the magnetic field. When the RF pulse is terminated, the nuclei return to their equilibrium state and the time required for this to occur is known as the relaxation time. The relaxation time consists of two parameters Known as spin-lattice (T1) and spin-spin (T2) relaxation and it is these relaxation measurements which give information on the degree of molecular organization and interaction of protons with the surrounding environment.
Since the water content of living tissue is substantial and variations in content and environment exist among tissue types, diagnostic images of biological organisms are obtained which reflect proton density and relaxation times. The greater the differences in relaxation times (T1 and T2) of protons present in tissue being examined, the greater will be the contrast in the obtained image [J. Magnetic Resonance 33, 83-106 (1979)].
It is known that paramagnetic chelates possessing a symmetric electronic ground state can dramatically affect the T1 and T2 relaxation rates of juxtaposed water protons and that the effectiveness of the chelate in this regard is related, in part, to the number of unpaired electrons producing the magnetic moment [Magnetic Resonance Annual, 231-266, Raven Press, N.Y. (1985)]. It has also been shown that when a paramagnetic chelate of this type is administered to a living animal, its effect on the T1 and T2 of various tissues can be directly observed in the magnetic resonance (MR) images with increased contrast being observed in the areas of chelate localization. It has therefore been proposed that stable, non-toxic paramagnetic chelates be administered to animals in order to increase the diagnostic information obtained by MRI [Frontiers of Biol. Energetics I, 752-759 (1978); J. Nucl. Med. 25, 506-513 (1984); Proc. of NMR Imaging Symp. (Oct. 26-27, 1980); F. A. Cotton et al., Adv. Inorg. Chem. 634-639 (1966)]. Paramagnetic metal chelates used in this manner are referred to as contrast enhancement agents or contrast agents.
There are a number of paramagnetic metal ions which can be considered when undertaking the design of an MRI contrast agent. In practice, however, the most useful paramagnetic metal ions are gadolinium (Gd.sup.+3), iron (Fe.sup.+3), manganese (Mn.sup.+2) and (Mn.sup.+3), and chromium (Cr.sup.+3), because these ions exert the greatest effect on water protons by virtue of their large magnetic moments. In a non-complexed form (e.g. GdCl.sub.3), these metal ions are toxic to an animal, thereby precluding their use in the simple salt form. Therefore, a fundamental role of the organic chelating agent (also referred to as a ligand) is to render the paramagnetic metal non-toxic to the animal while preserving its desirable influence on T1 and T2 relaxation rates of the surrounding water protons.
Art in the MRI field is quite extensive, such that the following summary, not intended to be exhaustive, is provided only as a review of this area and other compounds that are possibly similar in structure. U.S. Pat. No. 4,899,755 discloses a method of alternating the proton NMR relaxation times in the liver or bile duct of an animal using Fe.sup.+3 -ethylene-bis(2-hydroxyphenylglycine) complexes and its derivatives, and suggests among various other compounds the possible use of a pyridine macrocyclomethylenecarboxylic acid. U.S. Pat. No. 4,880,008 (a CIP of U.S. Pat. No. 4,899,755) discloses additional imaging data for liver tissue of rats, but without any additional complexes being shown. U.S. Pat. No. 4,980,148 disclose gadolinium complexes for MRI which are non-cyclic compounds. C. J. Broan et al., J. Chem. Soc., Chem. Commun., 1739-1741 (1990) describe some bifunctional macrocyclic phosphinic acid compounds. C. J. Broan et al., J. Chem. Soc., Chem. Commun., 1738-1739 (1990) describe compounds that are triazabycyclo compounds. I. K. Adzamli et al., J. Med. Chem. 32, 139-144 (1989) describes acyclic phosphonate derivatives of gadolinium complexes for NMR imaging.
At the present time, the only commercial contrast agents available in the U.S. are the complex of gadolinium with diethylenetriaminepentaacetic acid (DTPA-Gd.sup.+3 - Magnevist.TM. by Shering) and a DO3A derivative [1,4,7-tris(carboxymethyl)-10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclodode canato]-gadolinium (Prohance.TM. by Squibb). Magnevist.TM. and Prohance.TM. are considered as non-specific/perfusion agents since they freely distribute in extracellular fluid followed by efficient elimination through the renal system. Magnevist.TM. has proven to be extremely valuable in the diagnosis of brain lesions since the accompanying breakdown of the blood/brain barrier allows perfusion of the contrast agent into the affected regions. In addition to Magnevist.TM., Guerbet is commercially marketing a macrocyclic perfusion agent [1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecanato]gadolin ium (Dotarem.TM.) which presently is only available in Europe. Prohance.TM. is shown to have fewer side effects than Magnevist.TM.. A number of other potential contrast agents are in various stages of development.